Shothole Disease

• Plum (Prunus) tree leaves are susceptible to the development of Coryneum blight, a fungal disease that is also called shothole disease due to its propensity to produce what appears to be "holes" shot through the leaf. The first sign of infection is the growth of small, round, purple spots that eventually expand into brown or tan marks on the foliage. Those spots then die and drop out of the leaves, showcasing the "shothole" effect.
  
  

Management

• Measuring approximately 1/4 inch in diameter, the tan marks are the result of the fungus overwintering on the bark or buds of the tree, then spurting to life in the spring. The fungal spores are typically spread via warm spring wind and rain. Coryneum blight enjoys moisture, so avoid watering your flowering plum trees from overhead. Leaves, buds and twigs that are infected with shothole disease should be raked or pruned away and destroyed.
  
  

Damage

• Shothole disease is caused by the bacteria Xanthomonas arboricola pv. pruni and, as the holes weaken the leaves, causes the premature defoliation of a flowering plum tree. Twigs will develop sunken, dark lesions, and tan marks or brown spots often begin to dot the fruit. In some cases, a tan or brown gummy substance will ooze from the bark of twigs infected with Coryneum blight. Trees that are already weakened via wounds or stress are most susceptible to this disease.
  
  

Brown Rot

• Brown rot, caused by the fungus Monilinia fructicola, does not typically show itself on the leaves of a flowering plum, but it leaves tan marks and spots across much of the rest of the tree. It affects blossoms drastically, leaving them brown and hanging on the tree through summer. But the worst damage is saved for that of the fruit as green, immature plums develop heavy brown spotting followed by severe rotting of ripe, mature plums.